Production of low carbon ferroalloys

ABSTRACT

Reduction of the carbon content of ferro-alloys including stainless steel is effected by adding to the molten alloy a material more readily oxidizable than the alloy element and shattering a stream of the crude alloy by a jet of oxidizing gas, the temperature being thereby raised to a level at which carbon will be oxidized preferentially to the alloy element.

United States Patent 1191 Pearson [451 Oct. 1, 1974 1 PRODUCTION OF LOW CARBON FERROALLOYS [75] Inventor:

[22] Filed: May 18, 1971 [21] Appl. No.: 144,664

Related US. Application Data [63] Continuation-impart of Ser. No. 733,778, June 3,

1968, abandoned.

John Pearson, Dore, England [52] US. Cl 75/60, 75/129, 75/l30.5 [51] Int. Cl C22c 33/00, C2lc 5/28 [58] Field of Search 75/60, 129, 130.5

[56] References Cited UNITED STATES PATENTS 2,670,283 2/1954 Soisson 75/60 2,805,933 9/1957 Meyer et a1 75/60 2,819,160 1/1958 Bannister etal. 75/60 UX 2,915,380 12/1959 l-lilty 75/60 X 2,946,676 7/1960 Brennan 75/130.5 X 2,969,282 1/1961 Churcher... 75/60 3,003,865 10/1961 Bridges 75/60 3,198,624 8/1965 Bell et a1 75/60 X 3,218,157 11/1965 Dobrowsky et a1 75/60 3,236,637 2/1966 Mittermayr 75/60 3,325,278 6/1967 McClellan 75/60 3,336,132 8/1967 McCoy 75/60 X 3,374,088 3/1968 Epstein 75/60 3,393,997 7/1968 Faste 75/60 X 3,396,014 8/1968 Keyser l 75/130.5 3,672,870 6/1972 Rhydderch 75/60 FOREIGN PATENTS OR APPLICATIONS 817,783 8/1959 Great Britain 75/60 Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or FirmBrisebois & Kruger [5 7 ABSTRACT Reduction of the carbon content of ferro-alloys including stainless steel is effected by adding to the molten alloy a material more readily oxidizable than the alloy element and shattering a stream of the crude alloy by a jet of oxidizing gas, the temperature being thereby raised to a level at which carbon will be oxidized preferentially to the alloy element.

11 Claims, No Drawings PRODUCTION OF LOW CARBON FERROALLOYS This application is a continuation-in-part of our copending application Ser. No. 733,778 filed June 3, 1968, now abandoned.

This invention is concerned with improvements in and relating to the refining of ferro-alloys.

By ferro-alloy, we mean alloys of iron with elements such as chromium and manganese. Binary alloys will generally (but not always) consist of at least about 30 percent of the alloying element (i.e. chromium or manganese or the like). The ferro alloys will now usually contain less than 20 percent of alloying elements.

Ferro-alloys such as ferrochromium, ferromanganese and the like, used in particular for the de-oxidation and alloying of steel and iron, are made by a number of processes, e.g. by smelting of suitable ores in a blast furnace or electric furnace, or by an aluminothermic process. In the first two processes, the reducing agent used to convert the ore to metal is carbon whereas, in the latter, aluminium is the reducing agent. When carbon is the reducing agent, the ferro-alloys produced are contaminated to a greater or lesser extent with carbon which renders them less suitable for use in steelmaking, particularly for those steels where a low final carbon content is required. On the other hand, the aluminothermie process is more costly to operate and the low carbon ferro-alloys so produced are more expensive than those produced by the use of carbon.

For this reason, attempts have been made to reduce the carbon content of the cheaper ferro-alloys made by the carbon reduction. One of the methods employed has been to inject oxygen gas into or onto a bath of molten ferro-alloy in an endeavour to burn out the carbon. The method has not achieved such success because, until the bath has achieved a critical high temperature, the alloy element is oxidized in preference to the carbon. Although the heat generated by the oxidation of the alloy element eventually will raise the temperature of the bath to a level at which carbon is preferentially oxidized, so much alloy element is so consumed as to render the process uneconomical because of loss of yield or because the oxidized alloy element has to be reduced by some subsequent treatment in order to ensure a satisfactory alloy yield from the process. Moreover, the lower the carbon content desired, the higher must be the operation temperature and, thus, the more alloy element that is burned to produce this temperature. An additional disadvantage is that working a bath of metal at such high temperatures produces rapid wear on the refractories lining the treatment vessel.

We have now devised a method of reducing the carbon content of a ferro-alloy by which any loss of alloying element(s) can be avoided. In particular, the carbon content of the alloy can be reduced to very low levels, for example, 0.05 percent or less, without any loss of alloying element(s).

According to the present invention there is provided a method of reducing the carbon content of a ferroalloy of which the minimum temperature at which carbon is oxidized in preference to the alloying element(s) is below the vaporization temperature of the said element(s), which method comprises adding to the molten ferro-alloy an expendable material which is exothermically oxidizable, in preference to the alloy element(s), at temperatures from the temperature of the molten stream at least up to the said minimum temperature,

forming a molten stream of the ferro-alloy, at a temperature below the said minimum temperature. and shuttering the stream by oxidizing gas to form a multiplicity of droplets of the molten ferro-alloy, the quantity of said material in the stream being such that, upon oxidation thereof the temperature of the droplets is raised to a level at which the desired amount of carbon is oxidized in preference to the alloy element(s) but below that at which the alloy element(s) is/are vaporized, and the quantity of oxidizing gas being sufficient to oxidize said material completely and to oxidize the carbon in the droplets by the desired amount, and then collecting the molten droplets of ferro-alloy which droplets have a reduced carbon content and are substantially free from the expendable material.

The invention also includes a method of reducing the carbon content of crude stainless steel containing from 16 to 20 percent chromium and from 7 to 10 percent nickel, which method comprises adding to the molten crude stainless steel an expendable material which is exothermically oxidizable in preference to the chromium and nickel at temperatures from the temperature of the stream at least up to the minimum temperature at which carbon is oxidized in preference to the chromium and nickel, forming a molten stream of the crude stainless steel at a temperature below the said minimum temperature and shattering the stream by oxidizing gas to form a multiplicity of droplets of the molten steel, the quantity of said material in the stream being such that, upon oxidation thereof, the temperature of the droplets is raised to a level at which the desired amount of carbon is oxidized in preference to chromium and nickel but below that at which the chromium and nickel are vaporized, and the quantity of oxidizing gas being sufficient to oxidize said material completely and to oxidize the carbon in the droplets by the desired amount, and then collecting the molten droplets of the steel which droplets have a reduced carbon content and are substantially free from the expendable material.

The ferro-alloys which can be treated by the method of the invention are those in which carbon can be oxidized at a temperature below the vaporization temperature of the alloy element(s). With any particular type of ferro-alloy, such as ferro-chrome or ferromanganese, the minimum temperature at which carbon can be oxidized depends on the amount of carbon present. For example, with ferro-chrome containing percent chromium, the minimum temperature is about 2000C for 0.3 percent carbon, about 2200C with 0.1 percent carbon and about 2600C with 0.02 percent carbon. (A temperature of 2600C is too close to the vaporization temperature of chromium and hence carbon levels of 0.02 percent in a 70 percent chromium ferro-chrome cannot be obtained by the method of the invention. For similar reasons, ferro-vanadium cannot be decarburized by the method of this invention.) In all cases, as the quantity of carbon is reduced, the minimum temperature rises. It will be understood, therefore, that in operating the method of the invention, the amount of expendable material in the molten stream will be sufficient not merely to raise the temperature of the droplets to the minimum temperature appropriate to their initial carbon content, but also sufficient to raise their temperature to the minimum temperature appropriate to their final desired carbon content, due

allowance being made for the heat evolved during oxidation of the carbon.

It has been found that by methods according to the invention such operating temperatures of 2,200C and above can be achieved rapidly. By suitably adjusting the angle of the shattered stream and/or providing a suitably shaped reaction vessel and/or providing a flow of cool gas along the interior surface of the reaction vessel there are no deleterious effects on the apparatus due to the very high temperature materials present.

The expendable material present in the molten ferroalloy stream may, for example, be silicon and/or aluminium.

By way of illustration, to produce a low-carbon ferrochrome, crude ferrochrome containing 70 percent chromium is produced for example in the blast furnace or electric smelter. This ferrochrome may well contain several percent of carbon. The required amount of silicon or aluminium is then added. The molten crude ferrochrome is allowed to fall as a stream from a suitable nozzle and is subjected to the action of powerful jets of gaseous oxygen or oxidizing gases. These shatter the stream into fine droplets which, by presenting a large surface area to the oxidizing gas, bring about conditions for rapid reaction. In each drop, the first element to be oxidized is silicon or aluminium. The oxidation of silicon or aluminium will raise the temperature of the drops and once the minimum has been exceeded, carbon will be removed. Thus, the proportion of silicon and/or aluminium present in the ferrochrome must be such that the heat produced during its oxidation added to that produced by the proportion of carbon required to be removed is sufficient to achieve the final desired operating temperature. The treated liquid may then be caught in a suitable vessel whence it may be cast in conventional manner.

The amount of oxygen to be employed for the process may be calculated as that required to oxidize the silicon or aluminium and the desired amount of carbon.

Thus, a ferrochrome introduced into the spray process at a temperature of 1500C will need to be raised by the oxidation of silicon or aluminium and carbon to 2200C if it is desired to make an alloy containing not more than 0.1 percent carbon. This implies a temperature increase of 700C.

The increase in temperature of 700C c an be waste gases and the like. Such heat losses, as a proportion of the generated heat, decrease as the scale of operation increases but can never be fully avoided. More oxidizable material than in the examples quoted above must therefore be used in practice in order to ensure that the critical final temperatures are produced. The actual amounts are readily determined by plant operation with various proportions of added material. In a plant with moderate heat losses, the oxidation of 3 percent silicon or aluminium or appropriate combinations of silicon, aluminium and carbon have been found adequate to ensure a temperature rise of 700C. 7

' In order that the invention may be more fully understood, the following Examples based on experiments carried out and calculations made, are given by way of V illustration only.

EXAMPLE I To reduce to 0.10 percent the carbon content of a ferro-chrome containing 70 percent chromium, 2 percent carbon, 1 percent silicon, 0.03 percent phosphorus, 0.02 percent sulphur and balance iron 1.90 percent of the carbon has to be oxidized in the spray process together with sufficient silicon or aluminium to induce the desired temperature increase from l500C to 2200C. The necessary proportion of silicon or aluminium is achieved by adding 1.30 percent silicon as 75/25 ferro-silicon or 1.30 percent aluminium. This would require an addition of 40.1 lbs of 75/25 ferro-silicon or 29.57 lbs of aluminium for each ton of ferro-chrome.

The composition would then be:

a. 68.77 percent chromium, 1.96 percent carbon, 2.30 percent silicon, 0.03 percent phosphorus, 0.02 percent sulphur and balance iron, or

b. 69.09 percent chromium, 1.97 percent carbon,

0.99 percent silicon, 0.03 percent phosphorus, 0.02 percent sulphur, 1.30 percent aluminium and balance iron.

A stream of this ferro-chrome adjusted as above and issuing from a tundish nozzle at a rate of 20 tons per hour and at a temperature of 1500C is then subjected to jets of oxygen which shatter the stream, the quantity of pure gaseousoxygen (which for (a) issues at a flow rate of 28,600 ft/hr, and for (b) issues at a flow rate of 26,775 ft' /hr) required per ton of metal to be treated The bath of refined ferro-chrome would be at a temachievcd theoretically by the oxidation of 2 percent of perature of 1600C and would have a composition as silicon or aluminium (which is equivalent in terms of heat of combustion to the oxidation of5 percent of carbon). Appropriate combinations of silicon and aluminium will also (together with the carbon) produce the necessary temperature increase. In all metallurgical operations carried out at such high temperatures, there are inevitably heat losses by radiation, sensible heat of follows:

a. Chromium 71.76 percent, carbon 0.10 percent, phosphorus 0.03 percent, sulphur 0.02 percent,- balance iron (no detectable silicon) b. Chromium 72.09 percent, carbon 0.10 percent, phosphorus 0.03 percent, sulphur 0.02 percent, balance iron (no detectable silicon or aluminium).

EXAMPLE II To reduce to 0.10 percent the carbon content of a ferro-manganese containing 80 percent manganese, 1 percent carbon, 1 percent silicon, 0.20 percent phosphorus, 0.03 percent sulphur and balance iron, 0.90 percent of the carbon has to be oxidized, together with sufficient silicon or aluminium to induce the desired temperature increase from 1500C. to 2050C. The necessary proportion of silicon or aluminium is achieved by adding 1.50 percent of silicon as 75/25 ferro-silicon or 1.50 percent aluminium. This would require an addition of 46.37 lbs of 75/25 ferro-silicon or 34.05 lbs of aluminium for each ton of ferro-manganese. The composition would then be:

a. 78.38 manganese, 0.98 percent carbon, 2.50 percent silicon, 0.19 percent phosphorus, 0.03 percent sulphur and balance iron, or

b. 78.80 percent manganese, 0.98 percent carbon, 0.98 percent silicon, 0.20 percent phosphorus, 0.03 percent sulphur, 1.50 percent aluminium and balance iron.

A stream of ferro-manganese adjusted as above and issuing from a tundish nozzle at a rate of 20 tons per hour and at a temperature of 1500C is then subjected to jets of oxygen which shatter the stream, the quantity of pure gaseous oxygen (which for (21) issues at a flow rate of 22,570 ft/hr and for (b) issues at a flow rate of 20,310 ft lhr) required per ton of metal to be treated being:

'6 ious carbon contents in'ferromanganese and the like can be calculated bymethods given in the literature and known to those skllled in the art and the necessary operating conditions evaluated. It will be appreciated 5 that, in these cases, the temperatures required for prefcent and even below 0.03 percent. The conventional method for producing such low carbon stainless steel is to melt stainless steel scrap in an electric arc furnace and blow with oxygen to oxidize the carbon. This procedure suffers from the disadvantages described above that chromium is also oxidized before the carbon has been sufficiently removed and expensive low carbon deoxidizing alloys have to be added to reduce the chromium back from the oxide into the metal.

In proceeding in accordance with the invention,

' stainless steel scrap is melted and silicon or aluminium or a combination of these elements is added in sufficient amount so that, when the molten metal is submitted to the spray process as described above, the heat generated raises the metal to orabove the minimum temperature for the desired carbon content. Because The bath of refined ferro-manganese would be at a temperature of 1600C and would have a composition as follows:

a. Manganese 81.12 percent, carbon 0.10 percent,

phosphorus 0.20 percent sulphur 0.03 percent, balance iron (no detectable silicon) b. Manganese 81.55 percent, carbon 0.10 percent, phosphorus 0.20 percent, sulphur 0.03 percent, balance iron (no detectable silicon or aluminium) The minimum temperatures for other final carbon contents can be evaluated from information in the literature or by interpolation between the values given above. Experiments on eachparticular plant will indicate the amount of added oxidizable material to be added to reach the minimum temperature and the amount of oxygen to be used can be calculated by the method illustrated above. Lime may be added to the receiving vessel or as a powder alongside the falling stream of crude ferrochrome to flux the silica or aluchrome. The necessary minimum temperatures for varthe chromium content of the metal is lower than for' normal ferro-alloys, the minimum temperatures are also lower. For 0.1 percent carbon the minimum tem-.

perature is 1830C for stainless steel containing 18 percent of chromium and for 0.03 percent carbon it is 2000C Again based on experiments carried out and calculations made, the following specific example is given by way of illustration only of the production of stainless steel.

Example Ill 16 cwt of stainless steel scrap is melted in an electric arc furnce and sufficient ferro-silicon is added so that the composition is 0.42 percent carbon 6.0 percent silicon, 17.0 percent chromium, 8.85 percent nickel, 0.03 percent phosphorus, 0.02 percent sulphur, balance iron. This metal is transferred to the spray plant, flows from a tundish nozzle at a rate of 20 tons per hour and at a temperature of 1530C and is treated with 132 lb of oxygen and lb of lime, the oxygen flow rate being 41,250 ft"/hr. The temperature of the droplets rose to about 1970C. The final metal temperature is 1600C and the final metal composition is 0.04 percent carbon, trace silicon, 18.2 percent chromium, 9.45%

Ni, 0.03 percent phoshorus, 0.02 percent sulphur and balance iron.

There is no need to use molten stainless steel scrap. The crude ferro-alloy may be produced in a blast furnace or cupola, the requirement being that the molten metal shall contain the correct proportions of chromium and nickel in combination with other material more readily oxidizable than chromium, for the final operating temperatures appropriate to the desired final carbon contents to be achieved and that the amounts of oxygen used are sufficient to remove only the readily oxidizable material and the required amount of carbon.

Stainless steel and stainless steel scrap generally contains about 0.2% Si. although in exceptional circumstances the silicon content can be as high as 0.7 or even 0.8 percent. Even with such silicon contents, it is still necessary to add further silicon and/or another expendable material if no loss of chromium is to occur, even if the desired carbon content of the refined material is relatively high, for example 0.10 percent. For example, when a stainless steel melt (1600C) of composition 0.42% C, 0.7% Si, 17% Cr and 8.85% Ni is to be refined by the process of theinvention (but without adding any extra silicon) to a carbon content of 0.1 percent, the temperature of the droplets would need to reach 1830C and substantial loss of chromium would occur. The final composition of the refined steel would be 0.10% C, 16.7% Cr, 9% Ni, trace silicon and balance iron. This represents a loss of 0.5 lbs of chromium per 100 lbs of stainless steel melt, i.e. about 3percent of the chromium has been completely lost. This is a substantial amount when multi-ton batches of steel are involved.

The position is very much worse when it is desired to reduce the carbon content to a lower level, say about 0.04% C. If the steel. (at l600C) was refined to this carbon content (the temperature of the droplets would need to reach l970C) by the process of the invention (but omitting the addition of any expendable material), the final composition of the refined steel would be 0.04% C, 16.1% Cr, 9.2% Ni, trace silicon, balance iron. This represents a loss of 1.2 lbs of chromium per 100 lbs of steel or, put another way, a loss of about 7 percent of the chromium.

It will be understood, therefore, that if a stainless steel is refined by the process of the invention but without the essential step of adding an expendable material, loss of chromium will always occur and will be greater the lower the final carbon content of the refined steel. It is only by adding an expendable material that loss of chromium can be avoided. The same applies with ferroalloys and crude ferro-alloys such as ferro-chromium and ferromanganese. These materials do not contain sufficient expendable material (if they contain any at all) to avoid alloy element losses during refining. It is essential that expendable material be added to these materials before refining if no alloy losses are to occur.

What I claim is:

l. A method of reducing the carbon content of a ferro-alloy containing carbon and at least one alloying element such that the minimum temperature at which carbon is oxidized in preference to the alloying element(s) is below the vaporization temperature of the said element(s), which method comprises adding to the molten ferro-alloy an expendable material which is exothermically oxidizable, in preference to the alloy element(s), at temperatures from the temperature of the molten stream at least up to the said minimum temperature, forming a molten stream of the ferro-alloy, at a temperature below the said minimum temperature, and shattering the stream by oxidizing gas to form a multiplicity of droplets of the molten ferro-alloy, the quantity'of said material in the stream being such that, upon oxidation thereof the temperature of the droplets is raised to a level at which the desired amount of carbon is oxidized in preference to the alloy element(s) but below that at which the alloy element(s) become vaporized, and the quantity of oxidizing gas being sufficient to oxidize said expendable material completely and to oxidize the carbon in the droplets by the desired amount, and then collecting the molten droplets of ferro-alloy which droplets have a reduced carbon content but substantially their original content of said alloying element(s) and are substantially free fromthe expendable material.

2. A method according to claim 1 wherein the ferroalloy is ferro-chrome.

3. A method according to claim 1 wherein the ferroalloy is ferro-manganese.

4. A method according to claim 1 wherein the said expendable material is silicon.

5. A method according to claim 1 wherein the said expendable material is aluminium.

6. A method of reducing the carbon content of crude stainless steel containing carbon and from 16 to 20 percent chromium and from 7 to 10 percent nickel, which method comprises adding to the molten crude stainless steel an expendable material which is exothermically oxidizable in preference to the chromium and nickel at temperatures from the temperature of the stream at least up to the minimum temperature at which carbon is oxidized in preference to thechromium and nickel, forming a molten stream of the crude stainless steel'at a temperature below the said minimum temperature and shattering the stream by oxidizing gas to ,form a multiplicity of droplets of the molten steel, the quantity of said expendable material in the stream being such that, upon oxidation thereof, the temperature of the droplets is raised to a level at which the desired amount of carbon is oxidized in preference to chromium and nickel but below that at which the chromium and nickel are vaporized, and the quantity of oxidizing gas being sufficient to oxidize said material completely and to oxidize the carbon in the droplets by the desired amount, and then collecting the molten droplets of the steel which droplets have a reduced carbon content but substantially their original chromium and nickel content and are substantially free from the expendable rnaterial.

7. A method according to claim 6 wherein the expendable material is silicon.

8. A method according to claim 6 wherein the expendable material is aluminium.

9. A method of reducing the carbon content of a ferro-alloy containing carbon and at least'one alloying element such that the minimum temperature at which carbon is oxidized in preference to the alloying element(s) is below the vaporization temperature of the said element(s), which method comprises adding to the molten alloy, a material which is exothermically oxidizable, in preference to the alloy element(s), at temperatures from the temperature of the molten stream at least up to the said minimum temperature, forming a molten stream of the ferro-alloy at a temperature below the said minimum temperature, and shattering the stream by oxidizing gas to form a multiplicity of droplets of the molten ferro-alloy, the total amount of said expendable material in the stream being such that, upon oxidation of at least a part thereof the temperature of the droplets is raised to a level at which the desired amount of carbon is oxidized in preference to the alloy element(s) but below that at which the alloy element(s) become vaporized, and the quantity of oxidizing gas being sufficient to oxidize said added material completely and to COIL UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,839,018 Dated October 1, 197

Inv nt JOHN PEARSON It is certified that error appears in the above-identified patent and that said Letters Patentare hereby corrected as shown below:

[30] Foreign Application Priority Data June 8, 1967 Great Britain 26560/67 Signed and sealed this 3th day of April 1.975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademark FORM PO-1050 (10-69) uscoMM-oc 6O376-P69 0.5. GOVERNMENT PRINTING OFFICE IBIS 0-386-534. 

1. A METHOD OF REDUCING THE CARBON CONTENT OF A FERRO-ALLOY CONTAINING CARBON AND AT LEAST ONE ALLOYING ELEMENT SUCH THAT THE MINIMUM TEMPERATURE AT WHICH CARBON IS OXIDIZED IN PREFERENCE TO THE ALLOYING ELEMENT(S) IS BELOW THE VAPORIZATION TEMPERATURE OF THE SAID ELEMENT(S), WHICH METHOD COMPRISES ADDING TO THE MOLTEN FERRO-ALLOY AN EXPANDABLE MATERIAL WHICH IS EXOTHERMICALLY OXIDIZABLE, IN PREFERENCE TO THE ALLOY ELEMENT(S), AT TEMPERATURES FROM THE TEMPERATURE OF THE MOLTEN STREAM AT LEAST UP TO THE SAID MINIMUM TEMPERATURE, FORMING A MOLTEN STREAM OF THE FERRO-ALLOY, AT A TEMPERATURE BELOW THE SAID MINIMUM TEMPERATURE, AND SHATTERING THE STREAM BY OXIDIZING GAS TO FORM A MULTIPLICITY OF DROPLETS OF THE MOLTEN FERRO-ALLOY, THE QUANTITY OF SAID MATERIAL IN THE STREAM BEING SUCH THAT, UPON OXIDATION THEREOF THE TEMPERATURE OF THE DROPLETS IS RAISED TO A LEVEL AT WHICH THE DESIRED AMOUNT OF CARBON IS OXIDIZED IN PREFERENCE TO THE ALLOY ELEMENT(S) BUT BELOW THAT AT WHICH THE ALLOY ELEMENT(S) BECOME VAPORIZED, AND THE QUANTITY OF OXIDIZING GAS BEING SUFFICIENT TO OXIDIZE SAID EXPENDABLE MATERIAL COMPLETELY AND TO OXIDIZE THE CARBON IN THE DROPLETS BY THE DESIRED AMOUNT, AND THEN COLLECTING THE MOLTEN DROPLETS OF FERRO-ALLOY WHICH DROPLETS HAVE A REDUCED CARBON CONTENT BUT SUBSTANTIALLY THEIR ORIGINAL CONTENT OF SAID ALLOYING ELEMENT(S) AND ARE SUBSTANTIALLY FREE FROM THE EXPENDABLE MATERIAL.
 2. A method according to claim 1 wherein the ferro-alloy is ferro-chrome.
 3. A method according to claim 1 wherein the ferro-alloy is ferro-manganese.
 4. A method according to claim 1 wherein the said expendable material is silicon.
 5. A method according to claim 1 wherein the said expendable material is aluminium.
 6. A method of reducing the carbon content of crude stainless steel containing carbon and from 16 to 20 percent chromium and from 7 to 10 percent nickel, which method comprises adding to the molten crude stainless steel an expendable material which is exothermically oxidizable in preference to the chromium and nickel at temperatures from the temperature of the stream at least up to the minimum temperature at which carbon is oxidized in preference to the chromium and nickel, forming a molten stream of the crude stainless steel at a temperature below the said minimum temperature and shattering the stream by oxidizing gas to form a multiplicity of droplets of the molten steel, the quantity of said expendable material in the stream being such that, upon oxidation thereof, the temperature of the droplets is raised to a level at which the desired amount of carbon is oxidized in preference to chromium and nickel but below that at which the chromium and nickel are vaporized, and the quantity of oxidizing gas being sufficient to oxidize said material completely and to oxidize the carbon in the droplets by the desired amount, and then collecting the molten droplets of the steel which droplets have a reduced carbon content but substantially their original chromium and nickel content and are substantially free from the expendable material.
 7. A method according to claim 6 wherein the expendable material is silicon.
 8. A method according to claim 6 wherein the expendable material is aluminium.
 9. A method of reducing the carbon content of a ferro-alloy containing carbon and at least one alloying element such that the minimum temperature at which carbon is oxidized in preference to the alloying element(s) is below the vaporization temperature of the said element(s), which method comprises adding to the molten alloy, a material which is exothermically oxidizable, in preference to the alloy element(s), at temperatures from the temperature of the molten stream at least up to the said minimum temperature, forming a molten stream of the ferro-alloy at a temperature below the said minimum temperature, and shattering the stream by oxidizing gas to form a multiplicity of droplets of the molten ferro-alloy, the total amount of said expendable material in the stream being such that, upon oxidation of at least a part thereof the temperature of the droplets is raised to a level at which the desired amount of carbon is oxidized in preference to the alloy element(s) but below that at which the alloy element(s) become vaporized, and the quantity of oxidizing gas being sufficient to oxidize said added material completely and to oxidize the carbon in the droplets by the desired amount, and then collecting the molten droplets of ferro-alloy which droplets have a reduced carbon content, but retain substantially their original content of said alloying elements.
 10. A method according to claim 1 wherein the expendable material is a mixture of aluminium and silicon.
 11. A method according to claim 6 wherein the expendable material is a mixture of aluminium and silicon. 